The unique isozyme of lactate dehydrogenase (LDH) is found in mature testes and spermatozoa of most mammals and some birds and is termed LDH-X. The crystalline enzyme is highly immunogenic and interferes with normal reproductive processes in both males and females. The objective of this research is to determine the amino acid sequence of LDH-X from mouse testes so that the major antigenic determinants responsible for impairment of reproductive processes may be mapped. This can ultimately lead to the preparation of synthetic peptides containing these determinants and useful in contraception technology.